Wasn't it part of the BMI takeover conditions that a proportion of the LHR BMI slots had to be used for domestic purposes? That would explain a move from LGW for some services......
No. IAG has to make available slots to entrants who want to operate certain domestic (and other) routes where competition has been reduced (ABZ, EDI etc) but in terms of the slots BA gets it is free to do what it wants with them. However, Keith Williams has said the optimum use of slots is 1/3 long-haul and 2/3 short-haul so there is a lot of extra short-haul capacity to fill at LHR.